The next threat to ash trees in Europe

As you may know, Europe’s ash trees are at war with a fungus that causes the disease ash dieback. But even as the European ash (Fraxinus excelsior) fights this enemy, there is another threat that is already decimating ash trees in North America – the emerald ash borer beetle.

This pretty little insect – also known as Agrilus planipennis – is native to eastern Asia including China, Japan and the far East of Russia. The beetle lays its eggs on ash trees and the grubs burrow between the bark and the wood. This can destroy the phloem vessels and in severe cases kill the tree (1).

In 2002, the beetle was discovered in ash trees in Michigan, USA, and since then has been spreading across the US and into Canada (2). The beetle probably arrived in the US several years before it was first spotted and is said to have reached the continent in a shipment of Japanese car parts (1).

In eastern Asia, the local species’ of ash have evolved resistance and the presence of the beetle’s natural predators mean that the beetle causes little damage to the ash trees (3). However, none of the ash species in the North America are resistant and in the absence of natural predators the beetle populations have been exploding so that they can kill all the ash trees in an area within 11 years of arriving (1). Since arriving in the US, the beetle has killed tens of millions of ash trees (4).

Emerald ash borer beetle. Image by US Department of Agriculture (licensed under CC BY 2.0)

The beetle has reached the East coast of the US and with the level of trade across the Atlantic it is not inconceivable that the beetle could reach Europe. There are trade restrictions in place that reduce this risk, for example, it is illegal to bring ash logs with bark attached into the UK from areas that are known to have the beetle. Also, wood packaging — such as crates and pallets – from countries with the beetle must comply with the International Standard for Phytosanitary Measures with heat treatment or fumigation carried out in the exporting nation (5).

While it may be possible — if we are vigilant — to prevent the beetle from reaching Europe via the US, the beetle could also spread into Europe from Russia, a route that is far more difficult to control. Having reached Moscow, the beetle is now spreading west and south at a rate of up to 41 kilometres (25 miles) a year (4). While this doesn’t sound very fast, this could be speeded up by human activity as we trade wood and even live trees across Europe. Also, now that the beetle has reached the woodlands around Moscow, it will be difficult to track its advance across the almost continuous woodland that leads into Belarus and Ukraine.

It is promising that the European ash trees in Moscow appear to be showing more resistance to the beetle than their North American cousins (4). However, trees that are under stress are likely to be more vulnerable to the beetle and, since ash dieback disease is now widespread in many European countries, this does not bode well for ash. It may be that the trees that manage to fight off the ash dieback fungus may then die at the hands of the emerald ash borer beetle.

Researchers based in several European countries, including the UK, are studying ash dieback disease in the hope that the findings may help to boost the recovery of ash populations in the future. However, this research may be of little use if it does not also consider the effects of the emerald ash borer beetle. It is too late to protect the ash trees of Europe from ash dieback, but if we act now, we may be able to protect them from the beetle.

This post is dedicated to Professor Oliver Rackham OBE, botanist and “champion of woodlands” who passed away on the 12th February 2015 aged 75. His books completely changed the way I think about woodlands and, when I studied plant sciences at the University of Cambridge in the late 2000’s, he (and his lectures) had a legendary status amongst the undergraduates.